Global simulations of strongly magnetized remnant massive neutron stars formed in binary neutron star mergers

We perform a general-relativistic magnetohydrodynamics simulation for $\approx 30$ ms after merger of a binary neutron star to a remnant massive neutron star (RMNS) with a high spatial resolution of the finest grid resolution $12.5$ m. First, we estimate that the Kelvin-Helmholtz instability at merger could amplify the magnetic-field energy up to $\sim 1\%$ of the thermal energy. Second, we find that the magnetorotational instability in the RMNS envelope and torus with $\rho < 10^{13}~{\rm g~cm^{-3}}$ sustains magneto-turbulent state and the effective viscous parameter in these regions is likely to converge to $\approx 0.01$--$0.02$ with respect to the grid resolution. Third, the current grid resolution is not still fine enough to sustain magneto-turbulent state in the RMNS with $\rho \ge 10^{13}~{\rm g~cm^{-3}}$.Comment: 18 pages, 10 figures, PRD in pres

Jet collimation in the ejecta of double neutron star merger: new canonical picture of short gamma-ray bursts

The observations of jet breaks in the afterglows of short gamma-ray bursts (SGRBs) indicate that the jet has a small opening angle of < 10{\deg}. The collimation mechanism of the jet is a longstanding theoretical problem. We numerically analyze the jet propagation in the material ejected by double neutron star merger, and demonstrate that if the ejecta mass is > 10^{-2} M_{sun}, the jet is well confined by the cocoon and emerges from the ejecta with the required collimation angle. Our results also suggest that there are some populations of choked (failed) SGRBs or low-luminous new types of event. By constructing a model for SGRB 130603B, which is associated with the first kilonova/macronova can- didate, we infer that the equation-of-state of neutron stars would be soft enough to provide sufficient ejecta to collimate the jet, if this event was associated with a double neutron star merger.Comment: 6 pages, 3 figures, accepted for publication in the Astrophysical Journal Letter

Mass Ejection from the Remnant of a Binary Neutron Star Merger: Viscous-Radiation Hydrodynamics Study

We perform long-term general relativistic neutrino radiation hydrodynamics simulations (in axisymmetry) for a massive neutron star (MNS) surrounded by a torus, which is a canonical remnant formed after the binary neutron star merger. We take into account the effects of viscosity, which is likely to arise in the merger remnant due to magnetohydrodynamical turbulence. As the initial condition, we employ the azimuthally averaged data of the MNS-torus system derived in a three-dimensional, numerical-relativity simulation for the binary neutron star merger. The viscous effect plays key roles for the remnant evolution and mass ejection from it in two phases of the evolution. In the first $t\lesssim10$ ms, a differential rotation state of the MNS is changed to a rigidly rotating state, and as a result, a sound wave, which subsequently becomes a shock wave, is formed in the vicinity of the MNS due to the variation of the quasi-equilibrium state of the MNS. The shock wave induces significant mass ejection of mass $\sim(0.5-2.0)\times 10^{-2}M_\odot$ for the alpha viscosity parameter of $0.01-0.04$. For the longer-term evolution with $\sim 0.1-10$ s, a significant fraction of the torus material is ejected. The ejecta mass is likely to be of order $10^{-2}M_\odot$, so that the total mass of the viscosity-driven ejecta could dominate that of the dynamical ejecta of mass $\lesssim 10^{-2}M_\odot$. The electron fraction, $Y_e$, of the ejecta is always high enough ($Y_e\gtrsim0.25$) that this post-merger ejecta is lanthanide-poor; hence, the opacity of the ejecta is likely to be $\sim 10-100$ times lower than that of the dynamical ejecta. This indicates that the electromagnetic signal from the ejecta would be rapidly evolving, bright, and blue if it is observed from a small viewing angle ($\lesssim 45^\circ$) for which the effect of the dynamical ejecta is minor.Comment: 21 pages, 18 figures, accepted for publication in Ap